1. Field of the Invention
This invention relates to thin film electrical conductors and their methods of manufacture for use in integrated circuit application. More specifically, this invention relates to Al-Cu film electrical conductors for integrated circuit application.
2. Description of the Prior Art
Aluminum thin films are commonly used as the conductor metallization for integrated circuit applications. With trends towards smaller linewidths carrying larger current densities, electromigration failures have become a reliability issue in aluminum conductors. Small additions of copper to aluminum (&lt;4% by weight) have been found to increase electromigration lifetime by up to two orders of magnitude. However, the increase in lifetime is not consistently reproducible. The variability of electromigration behavior is the result of the microstructural inhomogeneity of the Al-Cu thin films which is not optimized to improve the electromigration behavior.
Al lines, made up from an Al-Cu alloy, are used as electrical interconnects for transistors, resistors, capacitors and all other components found within an integrated circuit (IC). The access time for an IC (the time within which it completes its designated task) typically goes down as the IC gets smaller and more densely packed. Power consumption and generated heat also go down as a device (IC) gets smaller. Strong market demand for energy efficient and faster integrated circuits is one of the driving factors behind smaller linewidth process technology. As the device density (i.e., 1 million transistors per chip) goes up, the acceptable line widths for the Al interconnects must be reduced to accommodate the higher device density. Under constant current, as an Al line gets thinner, the current density being carried by that line goes up. In other words, more electrons are forced to pass through an increasingly narrower cross section of Al. The electric field within the Al conductor creates a chemical potential gradient. This chemical potential gradient drives the process called diffusion which results in the movement of atoms within the conductor. The net movement of atoms in one direction results in a net movement of atomic vacancies (missing atoms in the crystal structure) in the opposite direction. Atomic diffusion occurs faster along defects in the crystalline material called grain boundaries. The increased atomic diffusion rate at grain boundaries is mainly related to the increased amount of free space at the boundary due to the mismatch of the crystal planes in adjacent crystals (grains). These vacancies can accumulate at certain parts of the microstructure and form voids which coalesce into larger voids which eventually can cause a break in the conductor. The movement of atoms and vacancies in this manner is termed electromigration and can result in failure of the IC.
Current state of the art has recognized that additions of copper to aluminum improve electromigration resistance. However, the mechanism by which copper alters electromigration behavior is unknown. Currently, copper is used in aluminum thin films as a "magic powder". Any thermal cycle to which the films are exposed in prior art practice are incidental to the process used to manufacture the integrated circuit rather than for the optimization of the thin film microstructure and therefore the conductive characteristics and, as such, are not necessarily optimized or consistently practiced to improve those characteristics. The present invention describes the mechanism for microstructural control and consequent electromigration improvement. The invention further details a process to microengineer the microstructure of aluminum-copper thin films to optimize electromigration resistance.
The present invention overcomes prior art shortcomings by providing a thermal treatment process designed to optimize the microstructure for electromigration resistance and therefore, behavior of Al-Cu thin films by engineering the thin film microstructure. In the as-deposited condition, the copper is present in the form of .theta.-phase Al.sub.2 Cu precipitates distributed randomly throughout the film both within the aluminum grains and at the grain boundaries. The formation of large .theta.-phase precipitates and a continuous coherent film of .theta.-phase precipitates at the aluminum grain boundaries impedes diffusion along the high diffusivity path in the thin film. As a result, the aluminum diffuse more slowly through the film and the electromigration lifetime is increased.
This is accomplished through a thermal treatment process that will cause the formation of large .theta.-phase precipitates and a coherent continuous thin layer of .theta.-phase precipitates to form on aluminum grain boundaries which slows aluminum diffusion during electromigration. The thermal treatment is in the range of 200.degree. to 300.degree. C. for up to 24 hours depending on temperature. This heat treatment causes the .theta.-phase precipitates inside the aluminum grains to dissolve and the coherent thin layer .theta.-phase precipitates at the grain boundaries to form.
The most important application of this invention is to inhibit Al diffusion along the thin film grain boundaries through enrichment of Cu at the Al grain boundaries. If the diffusion of Al atoms and vacancies is restricted along the grain boundaries, the resistance to electro-migration should be increased, thereby extending the life of the IC.